The present invention relates generally to an electrically conductive material adaptable particularly for a secondary battery and more particularly to an electrically conductive material comprising a polymer having conjugated double bonds, characterized by the use of a specific oxidizing agent comprising a cupric compound and a nitrile compound and a process for the preparation of the electrically conductive material, and to a secondary battery using this type of electrically conductive material.
It is known that polymers having conjugated double bonds in the main chain such as polyacetylene, poly-p-phenylene, polythienylene, polypyrrole, poly-p-phenylene-vinylene and polyaniline are remarkably improved in electric conductivity when they are treated with a P- or N-type doping agent such as arsenic pentafluoride, antimony pentafluoride, iodine, bromine, sulfur trioxide, n-butyllithium or sodium naphthalene, whereby they are changed from an insulator to a semiconductor or a conductor. These electrically conductive materials, so-called "electrically conductive polymers", are obtained in the form of powder, grain, bulk or film, which is used either as such or after molding thereof in accordance with the purpose of use thereof. They are now under investigation for application thereof to a wide variety of fields involving functional elements such as an antistatic material, an electromagnetic wave shielding material, a photoelectric conversion element (electron-light functional element), an optical memory (holographic memory) and various sensors; display element (electrochromism); a switch; various hybrid materials (transparent conductive film and the like) and various terminal equipment.
Among the above various electrically conductive polymers, polythienylene, polypyrrole and polyaniline are more stable in air than polyacetylene and thus hardly undergo oxidative deterioration, and are easily handled. Therefore, they are under investigations for application to various uses wherein these characteristics are effectively utilized.
Known process for the preparation of polythienylene, polypyrrole or polyaniline includes (1) electrochemical oxidation polymerization process (electrolytic polymerization process), (2) chemical oxidation polymerization process using an oxidizing agent and so on. According to the process (1), a film of polythienylene, polypyrrole or polyaniline is obtained by depositing polythienylene, polypyrrole or polyaniline in the form of a film on the anode used in the electrolytic polymerization and peeling it from the anode. According to the process (2), powdery polypyrrole is obtained by solid-phase, liquid-phase or gas-phase oxidation polymerization using an oxidizing agent, for example, peroxide such as potassium persulfate or ammonium persulfate, acid such as nitric, sulfuric or chromic acid or Lewis acid such as ferric trichloride, ruthenium chloride, tungsten chloride or molybdenum chloride. Further, it has been proposed in, for example, Mol. Cryst. Liq. Cryst. 1985, vol. 118, P.P. 149-153 that powdery polypyrrole similar to the one above obtained is obtained by oxidation polymerization using ferric perchlorate as an oxidizing agent in an organic solvent.
However, the above process (1) has disadvantages in that a film of polythienylene, polypyrrole or polyaniline is formed on the anode, so that the size of the film is restricted by the size of the electrode, which restricts the application of the process to mass production and that the electrolytic polymerization process itself is complicated, thus resulting in high cost.
On the other hand, although the process (2) is free from the disadvantages described above with respect to the process (1), it has other disadvantage in that the polythienylene, polypyrrole or polyaniline prepared by the process exhibits a so poor electric conductivity that the application field thereof is restricted.
Further, the process (2) wherein the oxidation polymerization is carried out in an organic solvent by using ferric perchlorate as an oxidant has a disadvantage in that the solubility of ferric perchlorate in an organic solvent is so much smaller than that in water that the application of the process to mass production is disadvantageously restricted, while the obtained polypyrrole exhibits a very low electric conductivity, because the concentration of a doping agent in the solvent is reduced by a decrease in the solubility. Additionally, the process has another disadvantage in that the obtained grainy polypyrrole comprises bulky primary particles having a diameter of 1 .mu.m or above. Therefore, the grainy polypyrrole causes scattering of dust and can not be easily handled in the following molding step, which is varied depending upon its use, owing to its small press molding density, so that the production of a high-density molded product from the polypyrrole is difficult to hardly obtain a material having a high electric conductivity. Still additionally, the process has another disadvantage in that various safety measures must be adopted in the production, since the process requires the use of an organic solvent which is in high danger of explosion or the like.
On the other hand, there has recently been proposed a secondary battery prepared by using an electrically conductive polymer comprising various organic materials as the electrode material.
Although such an electrically conductive polymer as used as an electrode material usually has a slight electric conductivity, the electric conductivity thereof can be dramatically increased by doping, since it can be doped with a dopant such as any one of various anions and cations, or can be undoped. In constituting a secondary battery with such an electrically conductive polymer as the electrode material, an electrically conductive polymer capable of being doped with anions is used as the anode material, and an electrically conductive material capable of being doped with cations is used as the cathode material, while such a solution containing a dopant as mentioned above is used as the electrolytic solution. Thus, there can be produced a secondary battery capable of charging and discharging via electrochemically reversible doping and undoping.
Known electrically conductive polymer of such kind as described above includes polyacetylene, polythiophene, polypyrrole and polyaniline. In an instance of polyacetylene, it is used as the electrode material for at least one of the anode and the cathode, while anions such as BF.sub.4.sup.-, ClO.sub.4.sup.-, SbF.sub.6.sup.- or PF.sub.6.sup.- or cations such as Li.sup.+, Na.sup.+ or R.sub.4 -N.sup.+ (wherein R represents an alkyl group) are employed to constitute an electrochemically reversible system capable of doping and undoping.
However, among this type of electrically conductive polymers, polyacetylene has a disadvantage in that it is very easily oxidized with oxygen in air in any state of doped and undoped states. Therefore, when polyacetylene is used as an electrode material, there have arisen such problems that the working atmosphere of electrode production must be controlled so severely that electrode production is difficult and complicated and that the obtained electrode itself is poor in preservability. Further, a battery using such an electrode as thus prepared has disadvantages in that the electrode is deteriorated or decomposed by the presence of only slight amounts of oxygen and water to lower battery characteristics, that the polymer tends to be deteriorated or decomposed by excessive charging and that the battery causes a rapid increase in charging voltage, a decrease in charging and discharging efficiency and a decrease in cycle life. Thus, polyacetylene is unsuitable as an electrode material.
Among the above various electrically conductive polymers, polythiophene, polypyrrole and polyaniline are more stable in air than polyacetylene and therefore hardly undergo oxidative deterioration, and are easily handlable. Therefore, when they are used as an electrode material of a battery, an electrode excellent in preservability can be easily produced without causing any of such disadvantages and problems are caused when polyacetylene is used.
However, the use of a film of polythiophene, polypyrrole or polyaniline prepared by electrochemical oxidation polymerization (electrolytic polymerization) has problems in that the production process is complicated and results in high battery production cost and in that the size and shape of the film are restricted by those of the electrode, since the polymer is formed on the electrolysis anode, so that it is difficult to mold the film into a required size which varies depending upon the kind of a battery. Further, since it is difficult according to the electrochemical oxidation polymerization to obtain a thick and uniform film with a high reproducibility, only a thin film thereof can be utilized as an industrial battery material. Thus, the use of such a thin film has a disadvantage in that the charging and discharging capacities of the electrode itself and the battery are so restricted that the enhancement of the capacities are nearly impossible.
On the other hand, although the use of polythiophene, polypyrrole or polyaniline prepared by chemical oxidation polymerization using an oxidizing agent is free from the above disadvantages, such a polymer exhibits a low electric conductivity, so that a secondary battery using the polymer as an electrode material causes ununiform charging and discharging reaction over the electrode with an increase in the internal resistance of the battery. Therefore, the charging voltage tends to be increased by repeating the charging and discharging cycles and the increased charging voltage causes decomposition of the electrolyte which disadvantageously leads to significant deterioration of the battery characteristics.
Further, polypyrrole prepared by chemical oxidation polymerization has a disadvantage in that the press molding density thereof can not sufficiently be enhanced, so that an electrode obtained by press molding the polypyrrole exhibits a low energy density owing to its low density. On the other hand, the production of a battery having a sufficiently high capacity by the use of an electrode made of the polypyrrole necessitates an electrode having an enlarged volume which is a barrier against miniaturization of a battery.